Sublime Text 3 how to change the font size of the file sidebar?

The answers are omitting the square brackets, in the case one is creating the file from scratch.

To recap, for the ST3 users who don't have the Default.sublime-theme file (which is actually the default configuration), the simplest procedure is:

1. Navigate to Sublime Text -> Preferences -> Browse Packages
2. Open the User directory
3. Create a file named Default.sublime-theme (if you're using the default theme, otherwise use the theme name, e.g. Material-Theme-Darker.sublime-theme) with the following content (modify font.size as required):

---

[
    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
        "font.size": 12
    },
]

For reference, here there is the full file (as found in ST2).

Ubuntu 18.04

Location of theme setting on Ubuntu 18.04, installed via sudo apt install sublime-text:

~/.config/sublime-text-3/Packages/User/Default.sublime-theme

MacOS

Location of theme setting on MacOS, installed via DMG:

~/Library/Application\ Support/Sublime\ Text\ 3/Packages/User/Default.sublime-theme

On Ubuntu, for versions of Sublime older than 3.2, what worked for me was changing the dpi scale in Preferences > Settings — User by adding this line:

"dpi_scale": 1.10 

For Sublime 3.2, you can use the following line instead:

"ui_scale": 1.10

Adjust the scale value as needed. After this change, you have to restart Sublime Text for it to take effect.